Three, Two, One: Impact
Impact Factor is often used as a measure of the importance of a journal. It is roughly a measure of how much recent articles are being cited in the journal. It has a number of well-known limitations.

For example,the amount of citation and number of papers published in different research areas varies greatly. This has been quantified, so if we compare the number of citations in engineering to mathematics, there are five times as many in engineering. There are 19 times as many in physics and there are around 80 times as many in medical areas. This means that the impact factors for journals in different areas can't be sensibly compared.

These sort of factors are largely well known. However, this article makes some interesting new points about impact factor. Impact factor is calculated by a private company, Thomson Scientific, based on data that they collect (for a fee). They then they publish the results (for a fee). The authors of the article tried to reproduce the calculations for impact factor based on data they purchased from Thomson Scientific, and found the numbers didn't match. They asked why and were given a second data set, which also did not reproduce the results. The authors comment on the second database:

This database appeared to have been assembled in an ad hoc manner to create a facsimile of the published data that might appease us. It did not.

They go on to say

It became clear that Thomson Scientific could not or (for some as yet unexplained reason) would not sell us the data used to calculate their published impact factor. If an author is unable to produce original data to verify a figure in one of our papers, we revoke the acceptance of the paper.

Strong stuff. The only reason Thomson have an edge over services like Google Scholar is because Thomson trade on their good reputation. It will be interesting to see if that is something that they can preserve.